156
FXUS63 KDMX 121733
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1233 PM CDT Mon Aug 12 2024

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Showers and a few storms through mid-morning, mainly central
  and south. Lingering isolated showers & storms into the
  afternoon. Severe weather not expected.

- Showers and storms likely midweek with potential for locally
  heavy rains. River flooding not anticipated.

- Return of more seasonal warmth and humid conditions by midweek

&&

.DISCUSSION...
Issued at 249 AM CDT Mon Aug 12 2024

After the end or reduction of this morning`s convection our
pattern will remain progressive highlighted by upper level
ridging through the MO Valley by Tuesday night, lowering heights
by midweek as a central/northern Plains upper trough matures
into the MS Valley by Friday morning, and then northwest flow
by the weekend as upper level ridging builds through the
Rockies. Currently PV anomalies near Omaha and more notably over
SD continue to provide both kinematic and thermodynamic support
for weak convection this morning. Much of the lift seems to be
driven by warm/theta-e advection associated with an H85/H7 front
and further aided by a nose of moisture into that layer`s baroclinic
zone. RAP soundings suggest the column is fairly saturated right
now, limiting elevated/MUCAPEs to no more than 500-750 J/kg
rooted 1-2km through this event. Effective shear does jump up to
~40kts at times, but 00z HREF updraft strengths suggest
instability will be insufficient to realize that shear and
limit any severe potential. 00Z HREF and 06z HRRR runs also
continue to highlight more appreciable rainfall accumulations
farther south into MO along the better 0-2km moisture
convergence. Much of the precip should move to the east by mid
morning, however isolated showers and a few storms may linger
into the afternoon in weak forcing and deeper moisture ahead of
the approaching eastern SD PV anomaly. Although much of the
precip should end by 00z, fog may develop tonight in lingering
low level moisture and weak southeast surface flow.

After a break Tuesday, our attention will then turn to more
precip potential from late Tuesday night through Thursday. While
model consensus for the middle of next week seemed to be
improving at this time yesterday, that does not appear to be the
case this morning with models again struggling on short wave
timing, strength, location, and of course similar questions with
associated surface features. However regardless of solution
there appears to be an active period with the most likely
scenario suggesting a theta-e advection driven MCS late Tuesday
night into Wednesday, and then a period more focused with a
surface wave sometime Thursday. This will also coincide with a
more summer-like airmass, with humid conditions as dewpoints
rise well through the 60s, and to 70F+ at times. Temps appear a
bit more tempered versus this time yesterday however, which may
limit instability somewhat. This and weaker flow aloft may keep
any severe weather potential relatively low, although not ruled
out.

Locally heavy rains would appear to be a somewhat greater
concern. 11/12z NAEFS and EC ensemble data have been consistent
depicting very anomalous low level specific humidities and
precipitable water values nearby Tue night through Thu with
percentiles well through the upper 90s. 00Z GFS depicts
precipitable waters ~2" with 4-4.5km warm cloud depths, all
suggesting efficient rainfall. 01Z NBM 50th percentile 48hr QMD
QPF also suggests widespread ~1.75" possible across the Mid MS
Valley Wed and Thu, which is notable for parameterized
convective precip. The max location is not in our forecast area,
but with model uncertainties with regard to details it is more
of a flag that locally heavy, efficient rainfall will be
possible in the vicinity. While rainfall may be appreciable,
recent dry conditions have tempered any river flood concerns
with only within bank rises suggested based on HEFS median
precip, and deterministic 120hr QPF. Ponding/standing water and
possibly a few flash flooding events would be more of a concern.

Most solutions show the parent upper low and associated surface
reflection to the east by Friday leaving us in northwest flow
aloft through the weekend and remainder of the period. The
pattern should mainly be dry with seasonal temps and humidity,
although spotty precip cannot be ruled out depending on the
strength of waves in the northern stream and northwest flow.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1233 PM CDT Mon Aug 12 2024

Lingering spotty showers across central Iowa this afternoon with
impacts at KDSM, including MVFR ceilings. Overnight fog is
expected with MVFR to IFR visibility and ceiling reductions
across most sites. While visibility is expected to improve
after 12-13z at most sites, lingering low stratus will continue
to impact sites through midday Tuesday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Small
AVIATION...Hagenhoff